1. Field of the Invention
The present invention relates generally to a synchronizing device and a synchronization method for a CDMA communication system, and in particular, to a synchronizing device and a method using spreading codes.
2. Description of the Related Art
FIG. 1 illustrates an IS-95 forward link of a base station, for transmitting channel signals to a mobile station in a Code Division Multiple Access (CDMA) mobile communication system. As shown, in a CDMA mobile communication system, the forward link includes a pilot channel, a sync channel and a paging channel. Though not illustrated, the forward link further includes a traffic channel for transmitting the voice and data of a user.
Referring to FIG. 1, a pilot channel generator 110 generates a pilot channel signal comprised of all xe2x80x9c1xe2x80x9d s for a pilot channel, and a multiplier 114 multiplies the pilot channel signal by an orthogonal code W0 to orthogonally spread the pilot channel signal. Here, a specific Walsh code is used for the orthogonal code W0. A multiplier 115 multiplies the pilot channel signal output from the multiplier 114 by a PN (Pseudo Noise) sequence to spread the pilot channel signal.
With regard to the structure of a sync channel generator 120, a coding rate R=1/2, constraint length K=9 convolutional encoder can be used for an encoder 121. A repeater 122 repeats sync symbols output from the encoder 121 N times (N=2). An interleaver 123 interleaves the symbols output from the repeater 122 in order to prevent burst errors. A block interleaver is typically used for the interleaver 123. A multiplier 124 multiplies the sync channel signal by a specific orthogonal code assigned to the sync channel to orthogonally spread the sync channel signal. The sync channel outputs the positional information, standard time information and long code information of the base station, and also outputs information for system synchronization between the base station and a mobile station. As stated above, the sync channel generator 120 encodes an input sync channel signal, and multiplies the encoded sync channel signal by a specific Walsh code Wsync assigned to the sync channel out of available Walsh codes to orthogonally spread the sync channel signal. A multiplier 125 multiplies the sync channel signal output from the multiplier 124 by the PN sequence to spread the sync channel signal.
With regard to a paging channel generator 130, an encoder 131 encodes an input paging channel signal. An R=1/2, K=9 convolutional encoder can be used for the encoder 131. A repeater 132 repeats the symbols output from the encoder 131 N times (N=1 or 2). An interleaver 133 interleaves the symbols output from the repeater 132 in order to prevent burst errors. A block interleaver is typically used for the interleaver 133. A long code generator 141 generates a long code which is the user identification code. A decimator 142 decimates the long code so as to match the rate of the long code to the rate of the symbol output from the interleaver 133. An exclusive OR gate 143 XORs the encoded paging signal output from the interleaver 133 and the long code output from the decimator 142 to scramble the paging signal. A multiplier 134 multiplies the scrambled paging signal output from the exclusive OR gate 143 by an orthogonal code Wp assigned to the paging channel to orthogonally spread the paging signal. A multiplier 135 multiplies the paging channel signal output from the multiplier 134 by the PN sequence to spread the paging channel signal.
As stated above, the orthogonally spread transmission signals of the respective channels are multiplied by the PN sequence to be spread, and up-converted into an RF (Radio Frequency) signal to be transmitted. In the IS-95 standard, spreading is performed using two different PN sequences for the I and Q arms. The PN sequences used herein have a period of 32,768.
In the forward link structure of FIG. 1, the pilot channel does not carry data and spreads a signal of all xe2x80x9c1xe2x80x9d s with a PN sequence of period 32,768 to transmit. In a system having a chip rate of 1.2288 Mcps (chips per second), one PN sequence period corresponds to 26.7 msec (80/3 msec). Upon power-on, the receiver in a mobile station acquires the pilot channel signal on the forward link shown in FIG. 1 in order to acquire synchronization with a base station.
FIG. 2 illustrates a receiver in a mobile station, which receives forward link channel signals from a base station.
Referring to FIG. 2, an RF receiver 212 receives an RF signal transmitted from a base station and then down-converts the received RF signal into a baseband signal. An analog-to-digital (A/D) converter 214 converts the baseband signal output from the RF receiver 212 to digital data. A searcher 222 acquires the pilot channel signal out of the forward channel signals in order to synchronize the mobile station with the base station. N fingers 231-23N despread corresponding forward channel signals to detect a correlation value among the channel signals. A combiner 226 combines the output signals of the respective fingers 231-23N.
As illustrated in FIG. 2, a receiver of a mobile station is comprised of the searcher 222, the N fingers 231-23N and the combiner 226. Acquisition of the pilot channel signal is performed by the searcher 222.
FIG. 3 is a timing diagram of forward channel signals that a base station transmits, in which the frame offset of a traffic channel is assumed to be 0.
Referring to FIG. 3, reference numeral 311 represents an 80 ms boundary of a base station, which is determined from a two-second boundary of the Global Positioning System (GPS). Reference numeral 313 represents the pilot offset of the base station. Reference numeral 315 represents the boundaries of three spreading sequence periods within 80 ms, from which it is clear that one spreading sequence period is 26.7 ms (80/3 ms). Herein, the spreading sequence is assumed to be a PN sequence. Each spreading sequence period is synchronized with a 26.7 ms frame boundary where a sync channel is interleaved. Here, the 80 ms frame will be referred to as the second frame and the 26.7 ms frame the first frame.
Reference numeral 317 represents an 80 ms frame boundary of the sync channel, while reference numeral 319 represents the frame boundaries of the paging channel and the traffic channel. For the traffic channel, the 80 ms frame is comprised of four 20 ms frames. Therefore, it is noted from FIG. 3 that in the 80 ms period, the sync channel is comprised of three 26.7 ms frames and the traffic channel is comprised of four 20 ms frames.
FIG. 4 shows the 80 ms frame structure of the sync channel. For the sync channel signal, the 80 ms frame, represented by reference numeral 412, is comprised of three 26.7 ms frames each including a sync bit SOM (Start of Message) set according to a pilot sequence period. For example, in the 80 ms period, the sync bit SOM for the first 26.7 ms frame period is determined as xe2x80x9c1xe2x80x9d (or xe2x80x9c0xe2x80x9d), and the sync bits SOMs for the following 26.7 ms frames are determined as xe2x80x9c0xe2x80x9d (or xe2x80x9c1xe2x80x9d). Therefore, detecting a sync bit SOM of xe2x80x9c1xe2x80x9d (or xe2x80x9c0xe2x80x9d) in the 80 ms period means detection of an 80 ms sync channel signal.
Referring to FIGS. 3 and 4, a description will be made regarding the synchronizing procedure performed between a base station and a mobile station. The standard timing of the base station is derived from the 80 ms boundary 311 which is determined from the two-second boundary of the GPS. The pilot channel signal of the base station is offset by the pilot offset 313 in the 80 ms boundary obtained from the GPS. This is to uniquely identify base stations using the same sequence by setting this pilot offset differently for each of the respective base stations. The pilot channel signals for the forward link are repeated at a period of 26.7 ms as represented by reference numeral 315. A sync channel signal is interleaved/deinterleaved at periods of 26.7 ms as represented by reference numeral 414, and this boundary is synchronized with one pilot sequence period (i.e., one PN sequence period). Therefore, upon acquiring a pilot channel signal, a mobile station in an IS-95 mobile communication system can accurately acquire the interleaving/deinterleaving frame sync for a sync channel as shown in FIG. 4. That is, the 26.7 ms period represents one PN sequence period (i.e., sync frame), and the 80 ms period represents a superframe period of a sync channel.
Thereafter, the mobile station should acquire the 80 ms boundary 317 of the sync channel. The sync channel for the forward link transmits the sync bit SOM every 26.7 ms as represented by reference numeral 414. The SOM bit is set to xe2x80x9c1xe2x80x9d in the first 26.7 ms frame and to xe2x80x9c0xe2x80x9d in the following two 26.7 ms frames. The receiver of the mobile station becomes synchronized with the 80 ms boundary utilizing the SOM bits of the sync channel. The receiver of the mobile station synchronizes with the pilot channel in order to be synchronized with the base station, whereby the receiver demodulates a signal on the sync channel every 26.7 ms, and determines a 26.7 ms frame with the demodulated SOM bit of xe2x80x9c1xe2x80x9d as the start of an 80 ms boundary.
The forward link structure of FIG. 1 and the synchronization procedure of FIGS. 3 and 4 are applicable to an IS-95 mobile communication system having a chip rate of 1.2288 Mcps. However, for high-speed data transmission and effective system design, an IMT-2000 system will increase the chip rate to use the wider bandwidth.
It is expected that the IMT-2000 mobile communication system will use a chip rate which is higher by 3, 6 and 12 times the chip rate of the existing IS-95 system. Herein, it is assumed that the chip rate of the IMT-2000 system increases to 3.6864 Mcps, three times the chip rate of the IS-95 system. In this case, if a PN sequence having the same period as that of a spreading sequence for the existing IS-95 mobile communication system is used, one PN sequence period decreases by 1/3 times to be 80/9 ms. In that case, the procedure for acquiring the 80 ms sync for the sync channel becomes complicated. In particular, even though the mobile station initially acquires a pilot channel signal, since it does not know the boundary of the 26.7 ms frame, it is not possible to use the sync acquiring procedure used in the 1.2288 Mcps narrow band system.
One method for solving this problem is to use a spreading sequence having a period which is as long as the increase in the chip rate. For example, when the chip rate is increased by three times, the period of the spreading sequence is also increased by three times so as to maintain one spreading sequence period to be 26.7 ms. However, the increase in length of the PN sequence by three times causes an increase in initial acquisition time of the mobile station.
Therefore, when the chip rate increases beyond that of the existing IS-95 system, a new initial synchronization method will be required.
It is, therefore, an object of the present invention to provide a device and method for rapidly performing initial acquisition and frame synchronization of a received signal at a receiver in a spread spectrum communication system.
It is another object of the present invention to provide a device and method for rapid frame synchronization for a data frame using a spreading sequence having the same frame boundary during spreading in a receiver for a CDMA communication system.
To achieve the above objects, there is provided a base station transmitter for a CDMA communication system including a superframe period of a sync channel, a plurality of first sync channel frames segmented from the superframe period, the first sync channel frames each having a first period, and a plurality of second sync channel frames segmented from the first sync channel frames, the second sync channel frames each having a second period, wherein sync channel signals are transmitted through the second sync channel frames. The base station transmitter comprises a circuit for generating the sync channel signals; and a channel spreader for channel spreading a sync channel signal in a leading sync channel frame out of the second sync channel frames in said each first sync channel frame with a first orthogonal code, and channel spreading sync channel signals in the remaining sync channel frames with a second orthogonal code.
A superframe of a sync channel as used herein refers to a frame for acquiring frame sync in an initial sync acquisition process, and it is assumed to be 80 ms in the embodiment. A first sync channel frame refers to segmented frames of the superframe. Here, it will be assumed that the superframe is segmented into three first sync channel frames. In this case, the first sync channel frame is 26.7 ms. A second sync channel frame refers to segmented frames of the first sync channel frame. It will be assumed herein that the first sync channel frame is segmented into three second sync channel frames. In this case, the second sync channel frame is 8.89 ms.
The present invention provides a CDMA communication system that uses a spreading sequence having the same length as the spreading sequence used in an IS-95 system in order to rapidly acquire synchronization, even though the chip rate increases.
To this end, a base station according to a first embodiment of the present invention spreads a specific second sync channel frame duration in a first sync channel frame duration with a first channel spreading code and spreads the remaining second sync channel frame durations with a second channel spreading code, wherein the second sync channel frames are spread using the short (i.e., high rate) PN sequence. Therefore, a mobile station can perform initial acquisition by initially determining a sequence having a high chip rate and a correlation value and thereafter rapidly acquire a boundary of a first sync channel frame transmitted from the base station, thereby to acquire frame sync.
According to a second embodiment of the present invention, a base station spreads a specific second sync channel frame duration with a first channel spreading code at a boundary of the superframe duration of the sync channel and spreads the remaining superframe duration of the sync channel with a second channel spreading code, wherein the second sync channel frames are spread using the short PN sequence. Therefore, a mobile station can perform initial acquisition by initially determining a sequence having the highest chip rate and a correlation value and thereafter rapidly acquire a boundary of the superframe of the sync channel transmitted from the base station, thereby to acquire frame sync.